Monday, 14 January 2002: 11:45 AM
Monthly mean extratropical atmospheric circulation response to El Nino SST
This study examines how well the NCAR CCM3 and CSM simulate the time dependence of the extratropical response to El Nino during winter seasons compared to observations. The composites of nine El Nino events during 1950-1994 using the NCEP data show that the 500 mb geopotential height over the Northern Hemisphere are significantly different between early winter (November and December) and later winter (January to March). The typical El Nino-related Pacific-North American (PNA) circulation pattern emerges in January and persist through February and March. The PNA patterns during these late winter months are coupled both with the El Nino-related tropical sea surface temperature (SST) and with the North Pacific SST. In contrast, the PNA patterns in the early winter months correlate only with the North Pacific SST. However, the CCM3 forced by the observed time-varying SSTs shows that the model circulation displays a PNA-like pattern in both early and late winter during El Nino years. Therefore, CCM3 overestimates the influence of tropical SST anomalies, but underestimates that of North Pacific SST anomalies on the Northern Hemisphere extratropical atmospheric circulation. The circulation patterns associated with El Nino in the CSM with atmosphere-ocean coupling are better than those in CCM3 in this regard.